Eduardo L Martin, Rafael Rebolo, Maria Rosa Zapatero-Osorio. American Scientist. Volume 85, Issue 6. Nov/Dec 1997.
How brightly a star shines throughout its life is determined largely by its birth weight: The greater its mass, the brighter it shines. But the relation has its limits: At the upper end of the weight scale, some extremely massive stars will ultimately collapse of their own weight to form a black hole-an object so compact that light cannot escape from it. At the lower end of the scale, a would-be star may not be sufficiently massive to ignite the thermonuclear reactions that make a star shine. In these instances the low-mass object that forms never reaches stardom, instead it sits quietly and dimly throughout its life as a substellar object, what astronomers now call a brown dwarf.
Originally called black dwarfs (and often called coffee dwarfs in Mexico), these substellar objects were first conceived of in the early 1960s as dark bodies floating freely in space. Stellar models had suggested that a true star must have a mass at least 80 times that of Jupiter to kindle the stable fusion of hydrogen. Objects with less than 80 Jupiter masses were believed to exist, but it was recognized that they would be extremely difficult to find because they would emit very little light.
Despite the appearance of some intriguing candidates over the years, it was not until 1995 that astronomers finally confirmed the existence of brown dwarfs with the discovery of two substellar objects-christened Teide 1 and Gliese 229B-which were reported within weeks of each other. As of August this year, at least half a dozen brown dwarfs have been confidently identified. The recent breakthrough has come from pushing the sensitivity of the detectors ever further with the new generation of large-format charge-coupled devices (CCDs)-sophisticated versions of the photosensitive arrays found in the ubiquitous videocamera. Near-infrared detectors in groundbased telescopes have also been improved, and these devices will continue to add to our discoveries. Because of these developments, previously unrecognized faint objects are now being discovered almost on a monthly basis.
The spate of new discoveries is allowing astronomers to compare the observed properties of brown dwarfs with those long supposed by the theorists. Aside from establishing that these objects do indeed exist, we have learned a little about the composition of browndwarf atmospheres, and we have been able to derive some preliminary estimates of their numbers and distribution in space. Although these studies are still at an early stage, the results have implications for models of stellar evolution (since brown dwarfs are, in some sense, “failed stars”) and cosmological theories (since some theories of dark matter suggest that substellar objects may harbor much of the “hidden” mass in the universe). Here we’ll provide some updates on the continuing search and discovery of these elusive objects.
Hunting for Brown Dwarfs
Brown dwarfs are curious celestial objects: They are neither planets nor stars, yet they share qualities with both kinds of entities. Theory suggests that a brown dwarf forms in the same way as a star-by the gravitational collapse of dust and gas in an interstellar cloudbut that its atmosphere may be vaguely reminiscent of a large gaseous planet, perhaps like one or more of the Jovian giants in our own solar system. Brown dwarfs are also relatively small objects: Although they may be nearly 80 times more massive than Jupiter, their diameters are similar to that of Jupiter, as gravity compresses the added mass into a denser object without an appreciable increase in girth.
Although brown dwarfs are dim compared to stars (with an intrinsic brightness less than one millionth that of our sun), they do emit a steady warm glow, owing to an atmospheric temperature somewhere between 300 and 3,000 degrees Kelvin. As a result, brown dwarfs are brightest at infrared wavelengths, and it seems plausible that they would indeed appear to have a deep, ruddy brown color to the naked eye. Some astronomers have even suggested that brown dwarfs may have “climatic” zones like the gaseous planets, giving these substellar objects a banded appearance, perhaps reminiscent of the colored latitudinal belts on Jupiter.
Because brown dwarfs have relatively low atmospheric temperatures, their outer layers should contain certain molecules that are otherwise “cooked” out of the atmospheres of hot stars. Notable among these are the oxides of titanium (10) and vanadium (VO). These molecules are destroyed by the energetic collisions in the hot gases of stars such as our sun (which has an atmospheric effective temperature of 5,770 degrees Kelvin), but they should dominate the spectra of brown dwarfs-they are the signatures of a cool object. Thus the brown dwarf’s cool temperature provides astronomers with a clue as to what they should search for-a faint object with certain molecules in its spectrum.
Unfortunately, nature makes the hunt for brown dwarfs a little tricky by throwing in a “red herring” in the form of an M dwarf star (or red dwarf)-a low-mass object that is the coolest true (hydrogen-burning) star, with a temperature below 3,500 degrees. As it happens, red dwarfs are believed to be the most common type of star in our Galaxy, perhaps numbering more than all other types of stars put together. To the naked eye they would appear to be a deep red color, and their diameters may not be much larger than a brown dwarf’s. Because they are so cool, red dwarfs also have titanium oxide and vanadium oxide in their atmospheres. Over the years, some of the best brown-dwarf candidates have turned out to be red-dwarf stars.
The telltale difference between brown dwarfs and red-dwarf stars arises from the absence of sustained thermonuclear reactions in the substellar object. This distinction permits the existence of an even more fragile substance-the element lithium-in the brown dwarf’s atmosphere. At the temperatures reached inside even the coolest true stars-about 2.5 million degrees-all of the lithium is destroyed. Because these stars efficiently mix the matter on their surfaces with that of their interiors, even a very old red-dwarf star with an atmospheric temperature resembling that of a brown dwarf will have destroyed all of its lithium. Brown dwarfs, on the other hand, never attain interior temperatures that can destroy lithium-provided their masses are less than 60 Jupiters. Brown dwarfs of 60 to 80 Jupiter masses do destroy lithium, and even burn some hydrogen, but these “transition objects” never become stable stars. Consequently, if lithium is seen in a very cool dwarf, it guarantees that it is a brown dwarf. In contrast, the absence of lithium is an ambiguous indicator of status since the object could be a star or a transition object.
This distinction would be meaningless if astronomers could not detect the lithium, but fortunately the element’s strongest atomic transition, the lithium-i resonance line, is located at visible wavelengths (670.8 nanometers) where the current CCDs have their peak sensitivity. Because of these traits the presence of atmospheric lithium has become the main tool for distinguishing between the smallest true stars (those at the bottom of the main sequence) and young brown dwarfs.
Knowing what to look for is only part of the challenge in finding a brown dwarf. Astronomers must also know where to look-after all, the universe is a big place, and as celestial objects go brown dwarfs are pretty small. Browndwarf hunters generally follow one of three strategies, each with its own set of drawbacks and advantages.
The strategy our research group uses relies on the assumption that brown dwarfs are brighter and hotter when they are first born. Consequently, it stands to reason that they should be easier to find in their youth. The place to find young star-like objects of any kind is in an open star cluster. Open clusters consist of stars so young that they have only recently begun to shine, perhaps only in the past few million years. Their clustering reflects the circumstances in which they were born-close to each other within a giant molecular cloud and where they are confined for the earliest parts of their lives. With time, the large-scale dynamics of a galaxy will inevitably scatter the stars of an open cluster, but while they are clumped together these young stars offer certain advantages to hunters of brown dwarfs.
Perhaps the most obvious advantage is that the components of an open cluster are restricted to a limited volume of space, so astronomers need only search a relatively small and well-defined area. Open-cluster searches offer an additional advantage in that the components share the same distance, age and chemical composition (or what astronomers call a star’s metallicity-the relative proportion of elements it contains that are heavier than helium). These qualities permit astronomers to compare star-like objects that differ only in their masses. (The Hyades and the Pleiades in the constellation Taurus are two of the closest open clusters, and they were the first two to be studied.)
Because of their specialized circumstances, the brown dwarfs in an open cluster may not be representative of all types of brown dwarfs. This is why wide-field searches are also necessary to find “free-floating” brown dwarfs. Indeed, most of our knowledge about the distribution of low-mass stars in the Milky Way comes from wide-field studies. Several populations of low-mass stars have been studied in different parts of our Galaxy, including those in the galactic disk near the sun, in the galactic bulge (near the center of the Milky Way) and in the galactic halo (the vast spherical area that surrounds the galactic disk). The low-mass stars in these different parts of the Galaxy differ from each other in their metallicities as well as their kinematics (the way they move through the Galaxy). Such differences may also exist among brown dwarfs. Unfortunately, it is as yet very difficult to infer the age of an isolated cool object, not to mention its distance and composition. Nevertheless, wide-field searches are necessary if we are to discover a lone brown dwarf close to our solar system, where it can be more easily observed.
A third approach in the hunt for brown dwarfs involves looking for the faint companions of true stars. In this approach, the brown dwarf may be detected directly or through its gravitational effects on the primary star. Since about half the stars in the sky are binary- or multiple-star systems, it seems likely that many brown dwarfs may be companions of true stars. This technique has the added advantage that some properties of the substellar companion can be inferred from the primary star, such as its distance and its metallicity. In some cases the orbital period might be short enough to derive an estimate of a brown dwarf’s mass from Kepler’s laws, which describe the dynamics between the components of the binary system. This may take several decades of observation about the duration of an astronomer’s career-but in principle it is feasible. As yet no dynamical mass is available for a brown dwarf.
The drawback of searching in binary systems is that the glare of the primary star makes it difficult to directly image the brown-dwarf companion. Also, companion brown dwarfs will not help us understand how isolated brown dwarfs may form in molecular clouds.
The Discoveries
Each of the three strategies used to hunt for brown dwarfs has met with success in the past two years. Teide 1 was discovered by the authors and our colleagues under the clear, dark skies of the island of Tenerife, Spain during a search for faint constituents of the Pleiades star cluster. In a scan of likely candidates, Teide 1 stood out because of its color and faintness, which suggested that it might be very cool. Measures of the object’s temperature and its atmospheric constituents-titanium oxide, vanadium oxide and neutral sodium (which indicates an object with a strong gravitational field)-gave away its identity. They were precisely those expected of a low-luminosity dwarf. Although we were confident that Teide 1 was a brown dwarf, the final confirmation of Teide l’s substellar status came with the detection of lithium in its atmosphere by our research group in collaboration with Gibor Basri and Geoff Marcy of the University of California, Berkeley. Ongoing surveys of the Pleiades cluster continue to detect even fainter brown-dwarf candidates that may be as small as 40 Jupiter masses. Spectroscopic confirmation of these objects is only a matter of months away.
The discovery of Gliese 229B was reported shortly after the discovery of Teide 1 by a collaborative team of astronomers at the California Institute of Technology and the Johns Hopkins University using the telescopes on Mount Palomar and the Hubble Space Telescope. This brown dwarf is a companion to a red dwarf star, Gliese 229A. The binary system lies about 20 light-years away, just close enough to be detected with a new instrument, the coronograph. This device hides the light of the star (in this case a red dwarf) behind a mask that enables a detector to see light from very faint objects located near the star. An infrared spectrum of Gliese 229B revealed the presence of methane (CH4), another fragile molecule that can only form at temperatures below about 1,500 degrees Kelvin. Gliese 229B is simply too cool to be a star.
The first discovery of a bona fide freefloating brown dwarf was reported last April by Maria Teresa Ruiz of the Universidad de Chile. Ruiz stumbled on the brown dwarf in the southern constellation Hydra during her search for white dwarfs (small, faint, but very hot and highly evolved stars). The objectnamed Kelu-1 (which means red in an ancient Mapuche language of central Chile)-is peculiar in that its spectral bands do not indicate the presence of titanium oxide or vanadium oxide, which are common in the coolest red dwarfs and some of the brown dwarfs that have been discovered. However, the identity of Kelu-1 was revealed when lithium and methane were found in its atmosphere. The presence of these fragile substances means that titanium oxide and vanadium oxide were not destroyed by heat, but that they must have condensed to form dust grains (such as perovskite, CaTiO3), which occurs at temperatures below about 2,500 degrees. Incorporated into dust grains, the spectral lines of these molecules are no longer seen.
Kelu-l’s low temperature indicates that it is probably older than the brown dwarfs found in the Pleiades cluster. Its relative dimness and the rapid rate at which it moves against the background stars suggests that this lone brown dwarf is quite close to our solar system, perhaps only 30 light-years away. Ruiz and her colleagues estimate that Kelu-1 is probably less than 75 Jupiter masses.
Teide 1, Gliese 229B and Kelu-1 represent merely the beginnings of a new field of study, which promises to become ever richer. Just this past July a number of new brown-dwarf observations were presented at the “Cool-Star” Conference in Boston. Among the more intriguing discoveries was the existence of two more free-floating brown dwarfs close to our solar system (within 50 light-years), which were confirmed by the presence of lithium in their atmospheres. They are estimated to be less than 60 Jupiter masses and less than one billion years old. These brown dwarfs were found in relatively narrow surveys of the sky, and an extrapolation to the whole sky suggests that there may be hundreds of brown dwarfs within a radius of only 50 light-years. Among the more unexpected recent findings is that one newly discovered brown dwarf is actually composed of two objects that orbit each other with a period of only a few days. This is the first known binary system that consists of two substellar objects. Further observations, scheduled in December, should provide more information about the pair.
How Many Brown Dwarfs?
Surveys are now being extended to larger parts of the sky to count brown dwarfs. It takes many nights of telescope time to survey the sky so deeply and many more nights to follow up on potential candidates, so it may be several years before the final results are in.
Nevertheless, preliminary findings suggest that brown dwarfs may be quite numerous, perhaps more common than stars like our sun.
The question is an important one that goes beyond merely an accounting of the Galaxy’s constituents. Brown dwarfs occupy an important place in cosmological discussions because they may contain a significant portion of the “dark matter” in our universe, the hidden mass that astrophysicists theorize must be present to account for the dynamic properties of rotating spiral galaxies and, on a much larger scale, the clustering of galaxies. Because brown dwarfs are made of everyday matter such as protons and neutrons (collectively called baryons), their cumulative mass contributes to the total amount of baryonic dark matter in the universe. (Weakly interacting massive particles, or WIMPs, are thought to comprise the nonbaryonic component of dark matter.) Jupiter-sized planets and black holes may also contribute to the total amount of baryonic dark matter in the universe.
As yet the relative abundance of brown dwarfs, Jupiter-sized planets, normal stars and black holes is simply not known. How many of each type of object there are depends on the distribution of masses during their formation from the condensing clouds of dust and gas that gave them birth. In astronomical jargon, this distribution is called the initial mass function, and it has remained unknown primarily because the dark objects are so difficult to detect. With the discovery of brown dwarfs in the Pleiades cluster, astronomers have begun a census below the substellar limit to compare the numbers of stars and substars formed in a typical cluster. Such censuses will go a long way toward determining the initial mass function and answering questions about the composition of dark matter.
To date, about 20 percent of the area of the Pleiades cluster has been surveyed at a level of sensitivity capable of detecting brown dwarfs. The results clearly indicate that brown dwarfs are quite numerous-they may comprise as much as 10 percent of the total mass of the Pleiades. (The preponderance of brown dwarfs in the Pleiades was recently confirmed with an extensive survey made with an 8,000×8,000-pixel CCD mosaic camera at the CanadaFrance-Hawaii Telescope on Mauna Kea, and sensitive survey in the center of the cluster made with the Isaac Newton telescope at La Palma, Spain.) Nevertheless, the accuracy of these estimates is still limited by an incomplete understanding of the formation of the lowest-mass brown dwarfs. Current ideas on the star-formation process link this minimum mass to the process of burning deuterium (2H), which is thought to be necessary to detach a developing object from the envelope of gas in which it forms. Deuterium burning does not take place in objects with less than about 12 Jupiter masses. If, as expected, brown dwarfs of less than 10 Jupiter masses are very rare, the entire population of brown dwarfs will contain much less mass than the whole population of true stars.
Much larger areas of the sky will need to be explored before astronomers know the answers to these questions. Two recently inaugurated surveys of the whole sky-2MASS (in the Northern Hemisphere) and DENIS (in the Southern Hemisphere)-offer the most promise as they are sensitive to infrared wavelengths where brown dwarfs are brightest. A few brown-dwarf candidates have already been reported by these groups. In June, a high-resolution spectroscopy image of one of the DENIS objects detected a strong lithium signature, confirming it as a nearby brown dwarf. However, even these surveys will miss the lowest-mass brown dwarfs (of 10 to 40 Jupiter masses) because these objects are very cool, emitting photons at long wavelengths beyond the sensitivity of these instruments.
Brown Dwarfs versus Planets
The discovery of brown dwarfs has occurred almost in parallel with the discovery of extrasolar planets. No one has yet obtained an image of an extrasolar planet, so we cannot compare them directly to brown dwarfs. Because extrasolar planets are even dimmer than brown dwarfs, astronomers only know of their existence by the gravitational effects they have on their parent stars, which appear to wobble periodically in the course of the planet’s orbit. This method of detection provides information on the planet’s minimum mass and its distance from the star, but no information about the nature of the planet’s atmosphere. As astronomers continue to find more brown dwarfs and extrasolar planets, we are bound to be challenged by a celestial body whose characteristics straddle the line between the two classes of objects. How will we tell them apart?
A comparison between brown dwarfs and a large planet in our own solar system illustrates some of the difficulties. For example, the planet Jupiter has an outer atmosphere dominated by methane and water, which also happen to be abundant on the brown dwarf Gliese 229B. The atmosphere of Teide 1, on the other hand, has little water or methane; its atmosphere consists mostly of carbon monoxide and the oxides of titanium and vanadium. Superficially, at least, Gliese 229B has more in common with Jupiter than it does with Teide 1. In this instance, Teide 1 stands out because of its tender age-a mere stripling of 100 million years-whereas Gliese 229B and Jupiter are about 10 to 50 times older. Models of brown dwarfs and planets suggest that both methane and water will accumulate in Teide l’s atmosphere as it ages. Knowing little else about these objects, they could be easily misidentified.
Some recent discoveries further confuse the issue. Two brown dwarfs, Kelu-1 and DENIS-P J1228-1547, differ not only from Jupiter, but from Gliese 229B and Teide 1 as well. These freefloating brown dwarfs have different molecules in their atmospheres and different colors. For the moment these enigmatic objects are challenging the theorist’s models, and the surprising variety of brown dwarfs has observers wondering how they will distinguish these substellar objects from planets as more of each type are found.
Several criteria have been proposed to make such distinctions, each with its own limitations. One of the simplest distinctions to make is on the basis of mass. Since objects smaller than 12 Jupiter masses cannot ignite the fusion of deuterium, some astronomers have suggested that evidence of deuterium burning would distinguish the majority of brown dwarfs and planets. Of course, this criterion will miss those brown dwarfs with less than 12 Jupiter masses and the occasional “super” planet that may be above this threshold. If one of these super planets were to escape its star with some of its moons, in effect forming its own planetary system, the object might be very difficult to identify Another distinguishing trait we might consider is the object’s orbital properties. Brown dwarfs are either free-floating bodies or companions to stars, whereas planets are typically associated with larger bodies (such as a star or a brown dwarf). Since the planets of our solar system have nearly circular orbits, it is assumed that an extrasolar planet would have a circular orbit as well, but a brown dwarf would have a more elliptical or eccentric orbit. This proposal also has its problemsfor example, what if a planet suffers an external perturbation and moves away from its circular orbit, gaining some eccentricity, or is even ejected from its parent star? Would it then become a brown dwarf instead of a planet?
Another way to discriminate between brown dwarfs and planets is by their internal structure. Simply put, brown dwarfs are thought to be the product of a collapsing cloud of interstellar gas. Planets, on the other hand, are believed to form by the agglomeration of smaller solid bodies (planetesimals) in a protostellar disk and to subsequently accrete gas from the surrounding medium. These processes should result in different internal structures, so that brown dwarfs would be chemically nondifferentiated, whereas planets would have a solid, highly metallic interior.
In practice, however, we cannot observe these processes for any single celestial object because of the enormous disparity between the time scales of these events and the comparatively brief careers of human astronomers. This criterion is the most difficult to implement. How are we going to probe inside these objects? The subject of brown dwarfs and extrasolar planets should be fertile ground for human ingenuity.
Finally, we should add that the differences among the known brown dwarfs suggests that the variation among these objects may rival the diversity we see among the planets. In particular, they may have fairly complex atmospheres, not all of which will be alike. Although most of the hydrogen should be “locked” in molecular form (H2), and the bulk of the carbon should be bound to oxygen to form carbon monoxide (CO), the excess oxygen need not form titanium oxide, vanadium oxide or water (H20). Clouds of dust particlesmade of corundum (A1203), perovskite (CaTiO3), iron, enstatite (MgSiO3) and forsterite (Mg2SiO4)-may be present, and they may profoundly modify the thermal structure and the density of a brown dwarf’s atmosphere. In some instances, changes in the spectrum of a brown dwarf seem to indicate changes in the object’s atmosphere-perhaps suggesting that they may have variable weather patterns (not unlike the gaseous planets). We expect more surprises as more of these objects are discovered.